1. Field of the Invention
This invention relates to the monitoring of a photographic bleach-fix regeneration cycle. In particular, it relates to the use of an electrochemical cell to sense -- and produce voltaic output based upon -- the electrochemical activity of ferrous and ferric ions in a sample solution compared to the activity of those ions in a reference solution.
2. Description of the Prior Art
Combined bleach-fix solutions are employed in processing silver halide photographic materials to accomplish bleaching and fixing in one step. These solutions incorporate a bleaching agent to oxidize the metallic silver in photographic elements to a soluble form, and a fixing agent to dissolve undeveloped silver halide and the silver salts formed by the action of the bleaching agent. Of particular importance -- because of their stability and their ability to achieve excellent results -- are bleach-fix solutions in which the bleaching agent is a ferric complex of an aminopolycarboxylic acid, and the fixing agent is a thiosulfate. Illustrative examples of aminopolycarboxylic acids can be found in U.S. Pat. No. 3,634,088 issued to Austin C. Cooley on Jan. 11, 1972. Ferric ion in such complexes converts metallic silver to ionic silver and is reduced thereby to ferrous ion.
So that spent bleach-fix solutions -- where the bleaching agent is a ferric complex of an aminopolycarboxylic acid -- containing ferrous ion and silver ion, may be restored to their original bleaching capability for reuse in the bleach-fix step of photographic processing, they are subjected to a regenerative treatment such as described in the aforementioned U.S. Pat. No. 3,634,088. In the first step, the solution to be regenerated is passed through a recovery vessel in which silver ion in the solution is chemically replaced by reduction using elemental iron (in a form such as steel wool, iron screen, iron flakes, and the like). The reaction giving rise to silver replacement is EQU Fe.degree. + 2 Ag.sup.+ .fwdarw. Fe.sup.+2 + 2 Ag.degree..dwnarw.I
silver being collected as a precipitate within the vessel. Concurrently, ferric ions (which are present in the spent solution because the bleaching agent is typically supplied in excess) react with the metallic iron within the recovery vessel to form ferrous ions by the following mechanism: EQU Fe.degree. + 2 Fe.sup.+3 .fwdarw. 3 Fe.sup.+2 II
the iron, of course, is exhaustible and demands eventual replacement. Otherwise collected silver is redissolved by ferric ions, and subsequently lost along with silver ions already in the solution, at great cost to the user. Precisely when the elemental iron is depleted may be estimated from the experience of previously exhausted recovery vessels. Or, one may observe a color change (going toward red) in the solution discharging from the recovery vessel, indicating the passage of ferric ions; had the elemental iron not been exhausted, ferric ions would have been reduced to ferrous ion (colorless or pale green in solution color). Both methods of detecting iron exhaustion are untimely, inaccurate and require a significant amount of personnel attention.
Because some lapse in time in discovering the exhaustion of the elemental iron is inevitable, one or more standby recovery vessels between the first vessel and the oxidation step (described below) are desirable. As the steel wool in the first vessel becomes exhausted, the unit next in line takes over. This procedure does avoid the loss of valuable silver; it regrettably does not provide an indication of when the first unit is spent. Until such indication is otherwise provided, silver replacement accomplished in the first unit is both reversed (by the dissolution of silver therein with ferric ions in the solution), and unnecessarily repeated by the standby unit next in line.
Whether one or more silver recovery vessels are employed, the inability to detect elemental iron exhaustion persists.
Step two in the bleach-fix regenerative cycle comprises treating the silver-free solution with an oxidizing agent, such as oxygen, halogen and the like, to convert ferrous ions therein to ferric ions, after which the ferric ion rich solution (to which various bleach-fix addenda have been added to compensate for dilution) is suitable for reuse in the bleach-fix processing region. Within the oxidizing region, unhindered supply of oxidising agent is essential. Nevertheless, experience has taught that the supply of oxidizing agent is often interrupted. For example, when air is employed as the oxidizing agent (as described in U.S. Pat. No. 3,634,088), it can be uniformly and rapidly bubbled into the solution through a porous head or sparger. If -- as they often do -- the sparger pores become plugged or the air compressor fails, curtailing the flow of air, too few ferrous ions in solution are oxidized to ferric ions. Spargers can also disintegrate. Although permitting adequate flow of air, this creates oversize bubbles with insufficient interfacial contact area between the air and the solution -- again causing insufficient oxidation. Early detection of either situation is difficult. Solution color, based on the predominance of either ferric or ferrous ions in the solution discharging from the oxidizing region, is, at best, a belated and poor indicator of insufficient oxidation.
A reliable method for accurately detecting when either the steel wool in a recovery vessel exhausts or insufficient ferrous-ferric oxidation occurs, entails the use of a conventional electrochemical sensing cell. The cell, which is responsive to the ratio of ferric to ferrous ions in the solution, senses and presents a display relating to changes of that ratio as either of the aforementioned problems are encountered.
The voltaic output of an electrochemical cell is derived from a comparison of the potentials of two adjacent half-cells. One half-cell is the sample solution to be measured; the other is a reference and comprises standardized solution of known or constant half-cell potential. The half-cells are in electrical communication with each other in one path by means of wire joining a sensing electrode in each solution. A second path of electrical communication is provided by a small aperture or sintered plug in a partition separating the solutions. Electrochemical sensing is well known to the art. In this regard, ferrous and ferric ions are referred to as an ion couple or redox pair. The terms half-cell, half-cell (redox) potential are founded in a mathematical relationship known as the Nernst equation for a half-cell reaction. A fuller understanding of these electrochemical terms can be found in U.S. Pat. No. 3,839,043 issued on Oct. 1, 1974 to W. Hunicke et al; Electro Chemical Science by Bockris and Drazic (Barnes & Noble, 1972); Introduction to Organic Electrochemistry by RiFi and Covitz (Marcel Dekker, Inc., New York 1974); and Orion Research Newsletter/Specific Ion Electrode Technology, Volume IV, Numbers 3 & 4 (1972, Orion Research, Inc.)
Commercially available electrochemical sensing devices provide a reference half-cell (including reference solution and electrode) but require that the user complete the cell by supplying sample solution in which to immerse an exposed test electrode. These devices are unfortunately expensive. Cost disadvantage, moreover, is aggravated by the tendency of ions to drift through the aperture or plug in the partition separating the half-cell solutions. Such drift occurs generally from regions of high concentration to low concentration causing the standardized reference solution to depart from its known or constant half-cell potential. Once more, even after such ionic drift, there continues to be a voltaic output from the cell which in and of itself does not forewarn the user of the inherent inaccuracy of such continued output. Eventually, a replacement unit with a new reference solution must be provided. In retrospect, it appears that the use of such an electrochemical cell is expensive in its equipment aspects, and gives rise to operating inaccuracies (stemming from the continued use of a unit having a reference solution of unknown half-cell potential).
Employing an electrochemical cell to sense ferrous-ferric ion ratios, nevertheless, is an attractive method of monitoring a bleach-fix regeneration cycle for problem detection. The monitoring method embodying the invention implements the electrochemical sensing principle and, moreover, avoids the expense associated with reference half-cells in commercially available electrochemical cells. The invention also provides in one embodiment a monitoring method which, though permitting the drift of ions between the sample solution and the reference solution is immunized against loss of constant half-cell potential normally caused by such drifting ions.